In recent years, improved output densities and energy densities have been required of electrochemical devices including electric double layer capacitors and lithium ion batteries.
Patent Document 1 below discloses an electric storage element wherein activated carbon is used in a positive electrode and a carbon material storing and releasing lithium ions is used in a negative electrode. This electric storage element is capable of higher voltage than conventional electric double layer capacitors and can thus increase energy density. In addition, this electric storage element is characterized by being capable of higher output than conventional lithium ion batteries and being excellent in cycle characteristic.
However, this electric storage element uses conventional activated carbon in the positive electrode, and thus the energy density thereof as an electrochemical device is limited by the performance of the positive electrode and is insufficient. The carbon material storing and releasing lithium ions is used in the negative electrode, and the potential of the negative electrode changes in the vicinity of potential at which lithium is precipitated. Accordingly, internal short-circuit is caused by precipitation of lithium dendrites, so that the risk of leading to accidents such as ignition and explosion cannot be eliminated.
Patent Document 2 below discloses an electric storage element wherein activated carbon is used in a positive electrode, lithium titanate is used in a negative electrode, and an organic solvent, a lithium salt and an ionic liquid are used in an organic electrolyte. The working potential of lithium titanium is about 1.5 V relative to the equilibrium potential of lithium (that is, vs. Li/Li+) and can thus attain higher safety than by carbon materials working near the potential at which lithium is precipitated. However, conventional activated carbon is used in the positive electrode, so the working potential of this electric storage element is about 2.5 V and is also low in capacity, thus making it impossible to expect advantage over conventional activated carbon capacitors.
Patent Document 1: Japanese Patent No. 3689948
Patent Document 2: JP-A 2005-353652